1. Field of the Invention
The invention relates to a twin spool turboprop unit having a construction which is well suited to control designed to maintain constant the speed of rotation of the propeller.
2. Summary of the Prior Art
Conventional twin spool turboprops usually comprise independently rotating low-pressure and high-pressure rotor groups or spools each comprising a compressor and a turbine which drives the compressor, the low-pressure compressor delivering at least partially to the high-pressure compressor, and the low-pressure turbine being disposed in series with the high-pressure turbine. The low-pressure rotor group or spool is drivingly connected to a variable-pitch propeller of the turboprop.
This construction is ill suited to constant-speed control of the propeller, since it entails a risk of the low-pressure compressor surging at low engine speeds. When the rate of fuel feed to the engine is low, for example when the aircraft is descending for landing, the rotational speed of the high-pressure compressor is reduced whereas the low-pressure compressor continues to run at high speed. This situation may cause unstable flow conditions in the compressors and the low-pressure compressor has a tendency to surge.